Vacuum solar thermal panel with radiative screen

ABSTRACT

A double-sided vacuum thermal solar panel comprising a vacuum-tight envelope ( 30 ) capable of withstanding atmospheric pressure when evacuated, said envelope ( 30 ) comprising a first and a second glass sheet ( 1, 2 ) transparent to solar radiation and facing each other, a perimetral frame ( 3 ) defining the lateral surface of said envelope ( 30 ), said solar panel comprising at least one first heat absorber ( 11 ), a second heat absorber ( 12 ), a pipe ( 13 ) which enters and leaves said envelope ( 30 ) by passing between said first and second heat absorber ( 11, 12 ), and a box element ( 10 ) which surrounds the outer surface of the pipe ( 13 ).

The present invention relates to a double-sided vacuum thermal solar panel in accordance with the introduction to the main claim.

Double-sided vacuum thermal solar panels are formed to absorb solar radiation from two active surfaces, one receiving sunlight directly and the other receiving sunlight reflected by a mirror. These are known for example from EP0387843. Said solar panels comprise a vacuum-tight envelope formed from two glass sheets transparent to solar radiation. One or more heat absorbers are disposed within the envelope to absorb the solar radiation by converting it into thermal energy. The heat absorber is generally a rectangular metal sheet of copper, aluminium or other metal of high heat conduction coefficient and low infrared emission coefficient, covered with a selective absorption covering highly absorbent to visible radiation but transparent to infrared radiation (for example chromium or nickel oxide). A pipe through which a thermo-vector fluid passes, generally water, enters and leaves the envelope and is in good contact with the heat absorbers, generally by a weld made along a generatrix of its outer surface. This weld is typically made by laser or ultrasound in order to minimize deformation of the metal sheet and damage to its covering. The thermal energy collected by the heat absorber is transmitted via said weld to the pipe by conduction, hence heating the fluid passing through it. One side of the double-sided solar panels receives the solar radiation directly onto each heat absorber, while on the opposite side the pipe, which lies in front of the heat absorber with which it is in contact, also receives part of the incident solar radiation. For this reason, in order to maximize performance, said selective absorption covering is also disposed on the outer surface of the pipe, the pipe normally also being made of copper to achieve low infrared emissivity.

Getter pumps are also positioned in vacuum panels, to absorb by a chemical effect any gas residues still present in the envelope after it has been evacuated, in order to maintain an adequate vacuum level for the entire operation period of the panel. These getter pumps normally consist of mass elements located within the vacuum envelope or of thin film coverings formed by the vacuum evaporation of a flash getter on an inner surface of said envelope. However they can also be advantageously formed as thin film coverings disposed on the surface of the heat absorbers under the selective absorption covering as described in EPI 706678.

A problem of current double-sided vacuum solar panels is that arranging a thin getter film on an absorber surface under the selective absorption covering negatively modifies the properties of said covering, by reducing its visible light absorption while at the same time increasing the infrared emissivity of the absorber.

Another problem is that the infrared emissivity coefficient of a surface provided with a selective absorption covering depends on the constituent material of said surface and not of the covering itself. Consequently, to limit the thermal energy losses by pipe irradiation, it is preferably made of copper, a material presenting a low infrared emissivity, but which considerably increases the panel production costs.

A further problem is that the weld between the pipe and absorber requires care in positioning said components as the contact surface area is very small, particularly in the case of laser welding in which the transverse dimension of the weld seam is very small.

An object of the present invention is therefore to provide a solar panel which enables the said drawbacks to be overcome and in which thermal dispersion due both to irradiation and to conduction are low.

A particular object is to provide a double-sided vacuum thermal solar panel comprising a getter pump which does not alter the absorption and reflection properties of the electromagnetic radiation by heat absorbers.

A further object is to produce the thermo-vector fluid pipe in a material less costly than copper, while at the same time limiting its infrared radiative losses.

Another object is to improve heat transfer between the heat absorber and the thermo-vector fluid flowing through the pipe, while at the same time facilitating their welding, particularly by laser.

Said objects are attained by a double-sided vacuum solar panel the inventive characteristics of which are defined in the claims.

The invention will be more apparent from the ensuing detailed description of one embodiment thereof, provided by way of non-limiting example and illustrated in the accompanying drawings, in which:

FIG. 1 is a perspective view of the solar panel of the invention;

FIG. 2 is an exploded perspective view of the solar panel;

FIG. 3 is a cross-section through the solar panel of the invention;

FIG. 4 is a perspective view of a box element traversed by the thermo-vector fluid transport pipe;

FIG. 5 is a perspective view of a part of the second heat absorber with is the transverse support element and the screening plates;

FIG. 6 is a cross-section through the solar panel at the transverse support element.

FIG. 1 shows a perspective view of the double-sided vacuum thermal solar panel. This solar panel comprises a vacuum-tight envelope 30 able to withstand atmospheric pressure when evacuated, comprising a first and a second mutually facing glass sheet 1, 2 transparent to solar radiation. The first and second glass sheet define the two active surfaces of the solar panel. Single-sided panels present a single glass sheet, while a second sheet facing the first can be made of metal. Double-sided panels present two facing glass sheets to increase thermal energy production, and in which the second glass sheet is traversed by solar radiation reflected by a mirror.

As can also be seen from the exploded view of FIG. 2, the vacuum-tight envelope 30 is bounded laterally by a perimetral metal frame 3. The perimetral frame 3 is joined to the glass sheets 1, 2 by a flexible metal belt 4, 5 joined to the perimetral frame 3 by welding, brazing or braze-welding and to the glass sheets 1, 2 by a glass-metal seal of mass type, in which the edge of the metal belt is embedded in the glass in accordance with the patent MI2008A 001245.

The first heat absorbers 11 are disposed within the envelope 30 to face the first glass sheet 1 such as to receive and absorb solar radiation which, by way of the first glass sheet 1, enters the envelope 30 directly. Second heat absorbers 12 are disposed again inside said envelope 30 but facing the second glass sheet 2 such as to receive and absorb solar radiation which enters the envelope 30 by way of the second glass sheet 2. These heat absorbers 11, 12 are rectangular metal sheets with their surface parallel to the glass sheets 1, 2. A selective covering, very absorbent towards visible light but transparent towards infrared, is disposed on those surfaces of said heat absorbers 11, 12 facing the glass sheets 1, 2. These heat absorbers 11, 12 are preferably made of copper provided with a selective absorption covering, as copper presents a high thermal conductivity and a low infrared emission coefficient. In this respect, the infrared emission by a selective absorption covering depends on the constituent metal of the surface, given that the selective absorption covering is transparent to infrared radiation. A covering transparent to visible light and reflective towards infrared is also disposed on that surface of the glass sheet 1, 2 facing the heat absorber 11, 12 within the envelope 30. In this manner the visible radiation originating from the sun passes through the glass sheets with low attenuation, to reach the surfaces of the heat absorbers 11, 12 where it is absorbed and transformed into thermal energy thereby, while at the same time the infrared irradiation of the heat absorbers, already very limited by the fact that these absorbers are made of copper and are covered by an infrared-transparent selective covering, is largely reflected by the covering disposed on the surface of the glass sheet 1, 2 within the envelope and is transparent to visible light but reflective to infrared, so further reducing losses to the outside.

One side of the perimetral frame is provided with two exit ports 20, provided such as to enable a pipe 13 to enter and leave the vacuum-tight envelope 30 with minimum heat transfer losses due to conduction, and a pumping port 19 for evacuating said envelope 30. The pipe 13 enters and leaves said envelope 30 by passing between said first and second heat absorbers 11, 12 and serves for transporting the thermo-vector fluid, generally water, which has to be heated in passing through the solar panel envelope 30. The outer surface of the pipe 13 is in contact with the first and second heat absorber 11, 12, which are generally welded along two opposing generatrices of the outer surface of the pipe 13, such as to create good thermal contact between the heat absorbers 11, 12 and the pipe 13, for easy thermal energy passage by conduction from the absorbers 11, 12 to the pipe 13 and to the fluid flowing through it. To increase the contact surface while at the same time facilitating welding and improving thermal conductivity between the heat absorbers 11, 12 and pipe 13, the pipe 13 can advantageously be of squashed shape at its centre, to form a flat contact surface 21, 22 with each of the heat absorbers 11, 12 (FIG. 3). This flat contact surface 21, 22 allows easier welding between the pipe 13 and the heat absorbers 11 and 12. It also allows better heat transmission between the heat absorbers 11, 12 and the pipe 13.

To limit dispersions by irradiation between the pipe 13 and the perimetral frame 3, which is at a lower temperature, the solar panel comprises box elements 10 surrounding the outer surface of the pipe 13, to form a radiative screen for the infrared radiation leaving it. The pipe 13, which generally after entering the envelope 30 is bent to form a serpentine coil, enters and leaves said box elements 10. In this manner the pipe 13, by passing within the box elements 10 which screen its radiative emission, can be made of materials other than copper, such as aluminium, which is decidedly less costly, even if having a much higher infrared emission coefficient, in particular at high temperature.

Said box elements 10 define an optically closed volume about the outer lateral surface of the pipe 13 and comprise said first and second heat absorbers 11, 12 with their edges 14, 15, 16, 17 bent to surround the outer lateral surface of the pipe 13, and a first and second plug 31, 32 for optically closing the base ends of the box element 10. In FIG. 2 it can be seen that these plugs 31 and 32 extend to also laterally screen the pipe 13. The edges 14, 15, 16, 17 of the heat absorbers 11, 12 are bent to obtain a substantially “C” cross-section for each individual heat absorber 11, 12 (FIG. 3). In this manner a first and second heat absorber 11, 12 together form the lateral surface of the box element 10. The edges 14, 15, 16, 17 of the first and second heat absorber 11, 12 are optically superposed relative to the outer surface of the pipe 13 to capture the thermal radiation emitted laterally by said pipe 13. An edge of each heat absorber could also be bent to provide the heat absorber with an “L” cross-section. Two coupled heat absorbers of “L” cross-section could form the lateral surface of the box element.

The box element 10 could present a lateral surface surrounding the pipe 13, but be open at the base ends due to the absence of the plugs 31 and 32. This would worsen the radiative dispersion, but could be justified by lower production costs for the solar panel. The getter pump formed by thin film covering is disposed on the inner surface of said box element 10, hence not interfering with the properties of the selective absorption covering disposed instead on the outer surface. Mass getter pumps of different type could also be inserted into the box element 10, for example in the form of pills or strips, which are disposed in contact with the inner surface of said box elements. Finally a flash getter can be evaporated onto the inner surface of the box element 10 by applying energy in the form of electromagnetic waves, so as to be deposited on the inner surface of the box element 10 following a command originating from the outside. None of these getter pumps interferes with the electromagnetic radiation absorption and emission properties of the heat absorbers 11, 12 as these are all disposed within the box element 10.

The envelope 30 is supported by a support frame 26 comprising longitudinal elements 27 and a transverse element 28. Said longitudinal elements 27 and transverse element 28 also serve to support the pipe 13 which forms a serpentine coil within said envelope. Several parallel pipes could also enter and leave the envelope 30 without forming a serpentine coil, in which case several exit ports 20 would be present.

FIGS. 5 and 6 show that the heat absorbers 11 and 12 present holes 18 through which the supports 24 for the pipe 13 pass without contacting the absorbers 11 and 12, locking pieces 33 being fixed by screws to the ends of these supports 24. These holes 18 are closed by copper plates 23 fixed to the supports 24 of the pipe 13, to screen the infrared radiation leaving the interior of the box elements 10 through said holes 18.

The absence of these plates 23 would worsen the efficiency of the solar panel overall, as the holes 18, although small, have the emissivity of a black body, equal to about twenty times that of a good selective covering disposed on a copper surface.

During operation the solar panel of the invention receives solar radiation, which passes through the glass sheets 1 and 2, is absorbed by the surface of the heat absorbers 11, 12 covered by the selective covering, and is transformed into thermal energy. This thermal energy is transferred to the pipe 13 mainly by conduction and then to the thermo-vector fluid within the pipe 13 by convection. Convection losses from the absorbers and pipe towards the outer walls of the envelope 30 are suppressed by the vacuum formed in the envelope 30, while irradiation losses from the pipe 13 are strongly limited by the box elements 10 which surround the various serpentine coil portions formed by the pipe 13 to act as radiative screens for it. The high vacuum within the envelope 30 is maintained with time by the getter pump disposed within the box element 10.

One of the advantages of the solar panel of the invention is that the getter pump disposed within the box element does not alter the infrared emission properties of the heat absorbers, so enabling the selective absorption coverings to be used effectively to optimise solar radiation absorption. Another advantage is that the radiation losses from the pipe are strongly limited by the presence of the box elements surrounding it to form a radiation screen therefore. This enables the pipe 13, used to transport the thermo-vector fluid, to be made of a material less costly than copper, for example aluminium, while still limiting radiation losses.

The flat contact surfaces 21, 22 of the pipe 13 enable the contact surface between the pipe 12 and the heat absorbers 11, 12 to be increased, so improving heat transfer to the thermo-vector fluid flowing through the pipe and at the same time facilitating its welding by laser.

Another advantage is that the box element can be easily and economically produced by bending the edges of the heat absorbers to “C” shape. 

1. A double-sided vacuum thermal solar panel comprising a vacuum-tight envelope (30) capable of withstanding atmospheric pressure when evacuated, said envelope (30) comprising a first and a second glass sheet (1, 2) transparent to solar radiation and facing each other to define the two active surfaces of the solar panel, a perimetral frame (3) defining the lateral surface of said envelope (30), said solar panel comprising at least one first heat absorber (11) disposed inside said envelope (30) and able to receive solar radiation through the first glass sheet (1), a second heat absorber (12) disposed inside said envelope (30) and able to receive solar radiation through the second glass sheet (2), a pipe (13) entering and leaving said envelope (30) by passing between said first and second heat absorber (11, 12) and presenting an outer surface in contact with said first and second heat absorber (11, 12), characterised in that said solar panel comprises inside the envelope (30) a box element (10) surrounding the outer surface of the pipe (13).
 2. A solar panel according to claim 1, characterised in that said box element (10) presents a lateral surface comprising said first and second heat absorber (11, 12).
 3. A solar panel according to claim 1, characterised in that said box element (10) defines a volume which is optically closed about at least a portion of the outer surface of the pipe (13).
 4. A solar panel according to claim 2, characterised in that said box element (10) presents two base ends and comprises a first and a second plug (31, 32) able to optically close said base ends.
 5. A solar panel according to claim 2, characterised in that said heat absorbers (11, 12) have edges (14, 15, 16, 17) which are bent such to define a substantially C-shaped cross section for each heat absorber (11, 12).
 6. A solar panel according to the preceding claim, characterised in that the edges (14, 15, 16, 17) of the first and second heat absorber (11, 12) are optically superimposed with respect to the outer surface of the pipe (13).
 7. A solar panel according to claim 1, characterised in that said pipe (13) presents on its outer surface a flat surface (21, 22) of contact with a heat absorber (11, 12).
 8. A solar panel according to claim 1, characterised by comprising a getter pump disposed inside said box element (10).
 9. A solar panel according to claim 1, characterised in that the box element (10) presents holes (18) for the passage of supports (24) adapted to support the pipe (13), said solar panel comprising small plates (23) able to shield the infrared radiation leaving from the holes (18).
 10. A solar panel according to claim 1, characterised in that said pipe (13) for transporting a thermo-vector fluid is made substantially of aluminium. 